This invention relates to a wavelength-division multiplexing optical transmission system which includes an optical multiplexer/demultiplexer.
Multiplexing optical transmission systems are effective in order to increase the transmission capacity of a single optical transmission line, and an exemplary one of various forms of such multiplexing optical transmission systems is a wavelength-division multiplexing optical transmission system. In a wavelength-division multiplexing optical transmission system, different kinds of light having different wavelengths are transmitted in the opposite directions or in one direction by way of a single optical transmission line. An optical multiplexer/demultiplexer is used to combine or separate light of different wavelengths with or from each other to effect transmission or reception of a signal in a wavelength-division multiplexing optical transmission system. As a form of such optical multiplexer/demultiplexer, there is an optical multiplexer/demultiplexer of the filter type which includes filters each formed from a dielectric multilayer film and provided on the opposite side faces of a parallelogram prism.
As a wavelength-division multiplexing module which employs such an optical multiplexer/demultiplexer of the filter type as described just above, a module of the bulk type is employed frequently wherein an optical multiplexer/demultiplexer of the filter type is placed on and secured to a board together with some other optical parts such as a laser diode and a photodetector. A wavelength-division multiplexing optical transmission system is already provided wherein such wavelength-division multiplexing modules of the bulk type as described above are provided on the transmission side and the reception side and connected to each other by way of an optical fiber.
When it is intended to transit light of three different signals having different wavelengths by means of a conventional wavelength-division multiplexing optical transmission system, it is necessary to form, as a filter to be formed on a side face of a prism of a multiplexer/demultiplexer, a band pass filter or a filter having a wide stopping band. However, since a band pass filter is expensive, there is a problem that the optical transmission system requires a corresponding high cost. Meanwhile, a filter having a wide stopping band is difficult to produce, and accordingly, there is a problem that also it leads a high cost of the optical transmission system.